About 7.5 percent to 20 percent of U.S. horses are geriatric horses, those greater than 20 years of age. In a survey of geriatric horses, it was noted that 7 percent are affected by some form of cardiovascular system disease.
Degenerative valve disease (DVD) is seen in geriatric horses and also horses as young as 10 years of age.
As horses age, something triggers the valves to leak blood backwards, usually affecting the aortic or mitral valves. The heart murmur typically is heard when blood starts to leak to the point where it generates noise.
"The literature would say that horses begin to be at risk in their teens for all valves, says Abby Sage, VMD, Dipl. ACVIM, University of Minnesota College of Veterinary Medicine.
The general consensus is that DVD is one of the more common cardiac diseases seen in horses, says Jerry Woodfield, DVM, Dipl. ACVIM, of Northwest Cardiology Consultants in Seattle.
"Fortunately when the murmurs are heard, many times they are not to a point that they are impacting the horse that dramatically. The concern is trying to get an idea of how risky any compromise of heart function is on anyone that is using the horse for athletic reasons," Woodfield says.
Photo 1: This image of regurgitation through the aortic valve was obtained from the left parasternal window in longitudinal plane in the left 5th ICS. The aortic valve is just above the base of the color-flow window.
"The significance of heart murmurs in horses is often difficult to determine if the horse is not exhibiting clinical signs or if the signs, such as poor performance, are non-specific," says Virginia Reef, VMD, professor and director of large animal cardiology at the University of Pennsylvania New Bolton Center.
Sources agree that valve disease is commonly an acquired disease. Throughout most of its life, the horse has a normal heart. As it ages, something triggers the valves to leak blood backwards, usually affecting the aortic or mitral valves.
"We don't know what the trigger is, whether it is mechanical force on the valve or a failure of some of the tissues. Outside of the rare case where you get an infected valve, otherwise the cause of the origination of the 'leaky valve' is unknown," Woodfield says.
Photo 3: This ultrasound image of a leaking mitral valve was imaged from the left side of the horse. Color-flow doppler was used to identify regurgitation or abnormal backflow (indicated by the arrows) of blood from the left ventricle to the left atrium, across a closed but "leaking" degenerative mitral valve. The size of the regurgitant jet and its effect on the size of the left atrium are useful in determination of a prognosis. LA is left atrium. LV is the left ventricle.